1. Field of the Invention
Besides nematic and cholesteric liquid crystals, optically active, tilted, smectic (ferroelectric) liquid crystals have also recently been used in commercial display devices.
2. Description of Related Art
Clark and Lagerwall have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in opto-electrical switching or display elements which have response times which are faster by a factor of up to 1000 compared with conventional TN (xe2x80x9ctwisted nematicxe2x80x9d) cells (see, for example, EP-A 0 032 362). Owing to this and other favorable properties, for example the possibility of bistable switching and the virtually viewing angle-independent contrast, FLCs are basically highly suitable for areas of application such as computer displays.
For a more detailed explanation of the technical requirements of FLCs, reference is made to European Patent Application 0 916 714 and DE-A 197 48 432.
Thiophene derivatives have already been described for use in liquid-crystal mixtures, for example in EP-B-0 364 923 or 0 500 072.
However, since the development, in particular of ferroelectric liquid-crystal mixtures, can in no way be regarded as complete, display manufacturers are interested in a wide variety of components for mixtures, partly because only the interaction of the liquid-crystalline mixtures with the individual components of the display device or of the cells (for example the alignment layer) allows conclusions to be drawn on the quality of the liquid-crystalline mixtures too.
It has now been found that tetrahydrothiophene derivatives of the formula (I), even when admixed in small amounts, have a favorable effect on the properties of liquid-crystal mixtures, in particular chiral smectic mixtures, for example regarding the dielectric anisotropy and/or the melting point, but also regarding the switching behavior, the tilt angle values and the temperature dependence of the tilt angle.
The invention therefore provides tetrahydrothiophene derivatives of the formula (I)
R1xe2x80x94Xxe2x80x94(A1xe2x80x94M1)axe2x80x94(A2xe2x80x94M2)bxe2x80x94A3xe2x80x94Yxe2x80x94Txe2x80x94Zxe2x80x94R2xe2x80x83xe2x80x83(I)
where the symbols and indices have the following meanings:
T is undirected and is tetrahydrothiophene-2,5-diyl or tetrahydrothiophene-2,4-diyl
R1 is hydrogen or a straight-chain or branched C1-20-alkyl or C2-20-alkenyl radical (with or without asymmetric carbon atoms), where
a) one or two nonterminal CH2 groups may be replaced, independently of one another, by xe2x80x94Oxe2x80x94 or xe2x80x94C(xe2x95x90O)xe2x80x94, with the proviso that two adjacent CH2 groups cannot be replaced in the same way, and/or
b) one CH2 group may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94, and/or
c) one CH2 group may be replaced by xe2x80x94Si(CH3)2xe2x80x94, cyclopropane-1,2-diyl, cyclobutane-1,3-diyl, cyclopentane-1,4-diyl, bicyclo[1.1.1]pentane-1,3-diyl or cyclohexane-1,4-diyl, and/or
d) one or more H atoms may be replaced by F and/or CN,
e) in the case of an alkyl or alkenyl radical containing asymmetric carbon atoms, the asymmetric carbon atoms have xe2x80x94CH3, xe2x80x94OCH3, xe2x80x94CF3, F, CN and/or Cl as substituents or are incorporated into a 3- to 7-membered ring, in which, in addition, one or two non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94 and one CH2 group non-adjacent to these groups may be replaced by xe2x80x94OC(xe2x95x90O)xe2x80x94;
R2 is hydrogen or a straight-chain or branched alkyl radical (with or without asymmetric carbon atoms) having 1 to 20 carbon atoms, where one nonterminal CH2 group may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94OC(xe2x95x90O)xe2x80x94 or xe2x80x94C(xe2x95x90O)Oxe2x80x94 and/or one or more H atoms may be replaced by F, with the provisos that
a) the CH2 group attached to Z cannot be replaced by xe2x80x94Oxe2x80x94 when Z is xe2x80x94Oxe2x80x94
b) R2 can only be hydrogen when Z is a single bond and R1 is not hydrogen
X is a single bond, xe2x80x94Oxe2x80x94, OC(xe2x95x90O)xe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94 or
Y is xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94SC(xe2x95x90O)xe2x80x94, xe2x80x94OCH2xe2x80x94 or xe2x80x94CH2CH2xe2x80x94
Z is a single bond or xe2x80x94Oxe2x80x94
A1, A2 and A3 are undirected and are each, independently of one another phenylene-1,4-diyl, unsubstituted or monosubstituted or disubstituted by CN or F, phenylene-1,3-diyl, unsubstituted or monosubstituted or disubstituted by CN or F, cyclohexane-1,4-diyl, in which one or two H atoms may be replaced, independently of one another, by CN, CH3 or F, 1-cyclohexene-1,4-diyl, in which one H atom may be replaced by F, 1-alkyl-1-silacyclohexane-1,4-diyl, pyridine-2,5-diyl, unsubstituted or monosubstituted by F, pyrimidine-2,5-diyl, unsubstituted or monosubstituted by F, thiophene-2,5-diyl or (1,3,4)-thiadiazol-2,5-diyl
M1 and M2 are undirected and are each, independently of one another xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94OC(xe2x95x90O)CH2CH2xe2x80x94, xe2x80x94OCH2CH2CH2xe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CH2CH2CH2CH2xe2x80x94 or a single bond;
a and b are each, independently of one another, 0 or 1.
xe2x80x9cTerminalxe2x80x9d means, for example in R1, the CH2 groups connected to X or to H. xe2x80x9cUndirectedxe2x80x9d means that incorporation of the group in the form of its mirror image is possible.
Preference is given to the following compounds of the formulae (I-1) to (I-1 5): 
in which:
R3 is hydrogen or a straight-chain or branched alkyl radical (with or without asymmetric carbon atoms) having 1 to 16 carbon atoms, in which, in addition, one nonterminal CH2 group may be replaced by xe2x80x94Oxe2x80x94 or, undirected, by xe2x80x94OC(xe2x95x90O)xe2x80x94 and in which one or more H atoms may be replaced by F;
R4 is hydrogen or a straight-chain or branched alkyl radical (with or without asymmetric carbon atoms) having 1 to 16 carbon atoms,
Fx denotes a substitution of the biphenyl by one or two (adjacent) F atoms.
Particular preference is given to compounds of the formulae (I), in particular (I-1) to (I-15), in which R3 and R4 are each, independently of one another, a straight-chain alkyl radical having 2 to 16 carbon atoms.
Particular preference is likewise given to compounds of the formula (I), in particular (I-1) to (I-15), in which R3 is a straight-chain alkoxy radical having 2 to 12 carbon atoms and R4 is hydrogen or a straight-chain alkyl radical having 2 to 12 carbon atoms.
Of the compounds of the formula (I) which are to be used as optically active components (dopants) in liquid-crystal mixtures, preference is given to those in which the alkyl group contains the asymmetric carbon atoms in the form of at least one of the following groups:
a) xe2x80x94C*H(CH3)CmH2m+1, where m has a value of from 2 to 8
b) xe2x80x94OC*H(CH3)CmH2m+1, where m has a value of from 2 to 8
c) xe2x80x94OC*H(CH3)CO2CmH2m+1, where m has a value of from 1 to 10
d) xe2x80x94OC(xe2x95x90O)C*H(CH3)OCmH2m+1, where m has a value of from 1 to 10
e) xe2x80x94OC(xe2x95x90O)C*H(F)CmH2m+1, where m has a value of from 1 to 10
f) xe2x80x94OCH2C*H(F)CmH2m+1, where m has a value of from 1 to 10
g) xe2x80x94OCH2C*H(F)C*H(F)CmH2m+1, where m has a value of from 1 to 10
h) oxirane-2,3-diyl
in which C* denotes the asymmetric carbon atom.
The compounds according to the invention are prepared by methods known per se from the literature, as described in standard works on organic synthesis, for example Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart.
However, it may prove necessary to vary or modify the literature methods for the requirements of mesogenic units, since, for example, functional derivatives having long ( greater than C6) alkyl chains tend to be less reactive than, for example, the methyl or ethyl analogues.
Particular reference is made in this connection to the following synthesis schemes, in which the synthesis of the tetrahydrothiophene derivatives of the invention is illustrated in more detail by way of example.
Scheme1 
i: 1. Na(Hg), H2O 2. HCl, C2H5OH as described in D. J. Gram, J.Am.Chem.Soc. 89, 4651 (1967).
ii: 1. NAOH 2. HCl
iii: R1xe2x80x94Xxe2x80x94(A1xe2x80x94M1)axe2x80x94(A2xe2x80x94M2)bxe2x80x94A3xe2x80x94OH, DCC/CH2Cl2 
The 5-alkylthiophene-2-carboxylic acids 1 required for the synthesis of Scheme 1 are prepared according to the method described in EP-B-0 364 923.
As far as the linking of functional derivatives of the tetrahydrothiophenes with other liquid-crystal-specific units is concerned, express reference is made to DE-A 197 48 432, which gives a list of methods customary to the person skilled in the art.
The invention further provides the use of compounds of the formula (I) in liquid-crystal mixtures, preferably smectic and nematic liquid-crystal mixtures, particularly preferably chiral smectic (ferroelectric) liquid-crystal mixtures. Particular preference is given to the use in ferroelectric liquid-crystal mixtures operated in inverse mode or in switching and display devices having active matrix elements.
Very particular preference is given to the use in mixtures for active matrix LCDs (switching and display devices) in which the chiral smectic liquid-crystal layer forms a monostable-switching monodomain.
The invention furthermore provides liquid-crystal mixtures, preferably smectic and nematic liquid-crystal mixtures, particularly preferably ferroelectric (chiral smectic) liquid-crystal mixtures, which comprise one or more compounds of the formula (I).
The liquid-crystal mixtures according to the invention generally comprise from 2 to 35 components, preferably from 2 to 25 components, particularly preferably from 2 to 20 components.
They generally comprise from 0.01 to 80% by weight, preferably from 0.1 to 60% by weight, particularly preferably from 0.1 to 30% by weight, based on the entire mixture, of one or more, preferably from 1 to 10, particularly preferably from 1 to 5, very particularly preferably from 1 to 3, compounds of the formula (I) according to the invention.
Further components of liquid-crystal mixtures which comprise compounds of the formula (I) according to the invention are preferably selected from known compounds having smectic and/or nematic and/or cholesteric phases. Further mixture components which are suitable in this context are listed, in particular, in international patent application WO 97/04039 and in DE-A 197 48 432, which are incorporated herein by reference.
The mixtures according to the invention can in turn be used in electro-optical or fully optical elements, for example display elements, switching elements, light modulators, elements for image processing and/or signal processing, or generally in the area of nonlinear optics.
The invention therefore furthermore provides a switching and/or display device preferably comprising a smectic liquid-crystal mixture, which comprises one or more compounds of the formula (I).
Particular preference is given to switching and/or display devices comprising active matrix elements (cf. e.g. DE-A 198 22 830).
The present application cites various documents, for example in order to illustrate the technical background to the invention. All these documents are incorporated herein by reference.